Display omitted •Using cotton as precursor to make sustainable lightweight microwave absorber.•Carbon-cotton was decorated by a flexible impregnation and annealing method.•Investigated the effect of temperature on the EMA properties of the media.•The RL value can reach −51.2 dB with bandwidth of 4.4 GHz at only 1.65 mm. Nowadays, carbon materials derived from natural biomaterials have been drawn considerable attention because the biomaterials are vastly available, accessible and renewable. Hereinto, it is fascinating that employing cotton as carbonaceous precursors to fabricate novel sustainable lightweight microwave absorber due to its unique hierarchical macro-/microporous architecture. In current study, we adopt a sustainable route from biomass cotton to construct cotton/zeolitic imidazolate framework (ZIF-67) and subsequent sintering to fabricate carbon-cotton/Co@nanoporous carbon (NPC) products. The as-prepared composites showed definitely superior microwave absorption performance than the pristine carbon-cotton and Co@NPC. It is confirmed that a better impedance matching is the key factor to realize excellent absorption. When the sample filling ratio is as low as 25 wt% in paraffin matrix, the maximum reflection loss can achieve −60.0 dB at 8.48 GHz. Furthermore, via adjusting the thickness to only 1.65 mm, the optimal reflection loss can obtain −51.2 dB at 13.92 GHz with a broad bandwidth of 4.4 GHz. In addition, the annealing temperature for the composites was also investigated, which had significant influence on tuning absorption properties. Prospectively, our present work may provide a new guideline to design carbon composites from earth-abundant recyclable biomass materials as sustainable, lightweight and high-performance absorber.